Digital over analog telephone lines Computers use digital pulses, but the public switched telephone network carries analog audio frequencies. Which device converts between digital and analog for transmission over phone lines?

Introduction / Context:
The classic telephone network was designed for analog voice, not digital data. To communicate over it, computers historically used devices that modulate digital signals into analog waveforms and demodulate them back into digital form at the other end. This is the origin of the ubiquitous term “modem.”

Given Data / Assumptions:

• Source: digital computer data.
• Channel: analog voice-band telephone line.
• Requirement: convert digital bits to suitable analog signals and back again.

Concept / Approach:
MoDem stands for MOdulator-DEModulator. The transmitting modem converts a digital bit stream into analog tones (using schemes like FSK, PSK, QAM). The receiving modem demodulates the analog signal back into the original digital bits, handling filtering, equalization, and often error control/handshaking.

Step-by-Step Solution:
Identify the mismatch: digital source, analog channel.Select the device that modulates/demodulates: the modem.Recognize ancillary functions: training, adaptive equalization, compression, error control.Confirm: device name is “modem.”

Verification / Alternative check:
Industry standards (e.g., ITU-T V-series) define modem operation over PSTN lines. Legacy dial-up connections on PCs used internal or external modems to achieve negotiated link rates.

Why Other Options Are Wrong:
Interface: too generic; does not imply modulation/demodulation.Attenuation: a channel impairment, not a device.Teleprocessor: vague term; not the standard name for the converter.

Common Pitfalls:

• Confusing modems with codecs; codecs convert between analog audio and digital PCM but are not necessarily for data modulation over PSTN.
• Assuming VoIP gateways are the same; they digitize voice but operate differently from dial-up data modems.
• Neglecting line quality issues that limit achievable data rates.

Final Answer:
Modem